Forming and shaping of thermoplastic materials, such as polyvinyl chloride (PVC) resin, generally requires heating a mass of the thermoplastic material to a temperature at which it can undergo plastic flow. Depending on the type of formation techniques used, this heated mass will be generally subject to shear stresses that form the mass into the desired shaped. In extrusion operations, for example, the heated mass is subsequently forced through a plate containing one or more orifice openings, sometimes referred to as a die plate, to impart a profile shape to the emerging extrudate. The extrudate is then cooled until it becomes sufficiently rigid to retain the profile shape. In calendering, the heated mass is forced into the nip between a roll and another object, such as a knife blade or other roll, and thereby sheeted into the desired shape and thickness.
In general a shaping feedstock composition is prepared by combining a thermoplastic polymer, such as PVC resin, with one or more adjuvants, such as lubricants. For example, extrudable PVC compositions are described in Handbook of Plastic Materials and Technology, Ed. I. Rubin, Wiley-Interscience, John Wiley & Sons, Inc. New York, 1990 and Plastics Additives and Modifiers Handbook, Ed. J. Edenbaum, Van Nostrand Reinhold, New York 1992, Chapter 3, each of which is incorporated herein by reference. Examples of additional components include filler, for example calcium carbonate, processing aids, for example acrylic copolymers, and stabilizer which imparts heat stability to the composition during the extrusion process, for example, a tin based stabilizer such as TM281® from Rohm & Haas. The composition is commonly then “fused”, that is, subjected to heating and blending, for example in a two mill roll, until it forms a plastic composition which is substantially homogeneous. The various additives and the amounts of the additives used influence the properties of the feedstock composition both during the fusion process and after it has been “fused.” The properties of the composition in turn affect the performance, such as processability, of the material during shaping, such as in extrusion and/or calendering or in follow-on processes, in which the feedstock material is employed. Of the various components comprising typical PVC feedstock compositions, the lubricant included in the composition can have an important influence on one or more of the properties related to processability, as well as on the quality of a the shaped article.
Dimensional stability is an important characteristic affecting the commercial value of a shaped product and hence of the processes and the components used to make the extruded product. Dimensional stability is assessed by observing the amount by which a shaped article expands during solidification after it emerges from the shaping apparatus or tool, such as the die plate in an extrusion operation or the calendaring nip in a calendering operation. The amount of expansion is sometimes referred to as swell or die swell.
For a given shapable feedstock composition, it is generally possible to increase the rate at which material is shaped. However, such shaping rate increases have practical limitations. For example, it is possible to obtain extrusion rate increases by operating the extruder at a higher temperature and/or at a higher head pressure. However, at some point of increasing extruder temperature the extrudable composition will generally begin to scorch, and the surface and structural qualities of the extrudate will begin to degrade. Similar effects occur in other shaping operations. Furthermore, as the amount of force used to shape the mass is increased, such as increasing the head pressure or extruder torque, a point is generally reached at which the dimensional stability and/or surface condition of the shaped material is unacceptable. These process limitations have created practical limits on the rate at which prior compositions could be extruded, calendered and the like.
Another mechanism that is at least theoretically available to achieve higher shaping rates is to increase the amount and/or type of lubricant incorporated into the formable feedstock composition. However, it is also possible that increasing the amount of the lubricant or changing the type of lubricant can have deleterious effects on the shaped product and/or on other aspects of the shaping process. For example, increasing the amounts of certain lubricants known as internal lubricant may compromise one or more the inherent physical properties of a shaped article, such an improvement in deflection temperature under load (DTUL) of the shaped article. Also, increased amounts of external lubricant may negatively impact not only the strength of the shaped product, but may also increase the fusion time, which increases the overall processing time.
It has been suggested that certain combinations of materials may be used as a lubricant in extrudable PVC compositions. For example, European Patent Application No. 79302422.5 discloses a PVC composition which incorporates as an alkaline earth metal or zinc soap as a stabilizer and mixed glycerides as a lubricant. The patent also indicates that sucrose mono- and di-esters may optionally included in the lubricant, in amounts ranging from about 20% to about 35% by weight of the composition. The patent does not disclose the acids from which the sucrose ester is formed, except to imply that they result from transesterification of sucrose with triglycerides. While it may be possible that some measure of success can be achieved in accordance with the teachings of European Patent Application No. 79302422.5, applicants have discovered that certain types of sucrose esters are highly undesirable from a performance standpoint, from a cost standpoint, or both.
Applicants have thus come to recognize the need for formable or shapable compositions, particularly PVC-based compositions, having one or more of those desirable shaping characteristics associated with high levels of lubricant while reducing or substantially eliminating one or more or the adverse effects that would otherwise be associated with a high level of lubricant. The present methods, feedstock compositions and shaped articles meet this and other needs.